[PATCH for-7.5 4/5] memcg: avoid queuing high_work from reclaim

[Shakeel Butt]

Kernel threads and remote chargers use high_work when memory.high is
exceeded. Their allocations during reclaim can also charge memory,
for example when zswap stores a page.

Without a check, the high_work worker can queue itself from its own
reclaim. The pending bit is cleared before high_work runs, so each run
can queue another run.

Skip memory.high enforcement for charges from task context with
PF_MEMALLOC set when they would use high_work. This covers kernel
threads and remote charges. The memory is still charged. Interrupt
charges keep their current path.

In a VM with incompressible zswap data, the worker queued itself 163 times
without this check.

Signed-off-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>
---
 mm/memcontrol.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 3456960cbe68..d1e022e6572f 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2708,6 +2708,14 @@ static void memcg_enforce_high(struct mem_cgroup *memcg, unsigned int nr_pages,
 	bool use_worker = !in_task() || (current->flags & PF_KTHREAD) ||
 		!current->mm || !mm_match_cgroup(current->mm, memcg);
 
+	/*
+	 * Reclaim can charge memory, for example when zswap stores a page.
+	 * Don't queue high_work from a reclaiming task, or the
+	 * high_work worker could keep queuing itself.
+	 */
+	if (in_task() && use_worker && (current->flags & PF_MEMALLOC))
+		return;
+
 	/*
 	 * If the hierarchy is above the normal consumption range, schedule
 	 * reclaim on returning to userland.  We can perform reclaim here
-- 
2.53.0-Meta